SPHERES Development and Demonstrations of Close Proximity Formation Flight Maneuvers

The Synchronized Position Hold Engage Reorient Experimental Satellites (SPHERES) program, developed by the MIT Space Systems Laboratory, began operations aboard the International Space Station (ISS) in May 2006. SPHERES was designed as a research facility to demonstrate metrology, control, and autonomy algorithms for distributed satellite systems. By operating in the risk-tolerant environment of the ISS, SPHERES allows researchers to push the limits of their algorithms. The motivation for SPHERES formation flight tests arises from the desire to develop fractionated spacecraft, which use loosely coupled formation flight to maintain multiple satellites in close proximity to achieve a common goal. Fractionated spacecraft allow better adaptability, survivability and payload isolation than a large single-unit spacecraft. The system may consist of heterogeneous spacecraft (e.g., only one high-power communications satellite for ground telemetry, while all other use lowpower inter-satellite communications) or homogenous spacecraft (e.g., all identical satellites are part of a sparse aperture, reconfigurable space based radar). The nature of these designs does not require precision formation flight, but rather requires coarse control to maintain a loose formation where the critical elements are to stay within range of each other and maintain a safe enough distance to prevent collisions and obstructions. This paper presents a method to remove the global navigation sensors for all but one satellite in the fractionated spacecraft system in order to reduce system cost and complexity. In this problem, all satellites must follow a globally defined trajectory in order to avoid collision with another object. Several tradeoffs and limitations associated with this approach and are discussed in this paper.